The present invention relates to an input coupling circuit for a measuring instrument such as a multimeter, and particularly to such a circuit adapted to decouple an input terminal or terminals from a low impedance path when a voltage is to be measured.
Measuring instruments are available which are multipurpose, e.g. that can provide voltage or resistance measurements at the same set of input terminals or probes without requiring manual switching from one mode of operation to another. Such an instrument typically includes a comparison circuit that will automatically change measurement functions from ohms to AC or DC volts if a voltage signal larger than a predetermined value is detected. This feature is intended to render the meter quicker and easier to use.
For ohms measurements, the multimeter needs to source about 0.5 to 1 milliamperes of DC current from a 1 to 3 volt source. In order to accomplish this result, the resistance in the meter circuit has to be less than approximately 10,000 ohms. However, the same circuit must withstand the maximum rated input voltage of the multimeter, typically in the range of 600 to 1000 volts. A 10,000 ohm resistor in series with the circuit would drop this voltage, but the power rating required would result in a physical resistor size prohibitively large for a hand-held instrument. Furthermore, voltage measuring instruments are conventionally high impedance devices in order to maintain measuring accuracy and avoid loading the circuit under test. A much higher impedance than a few thousand ohms is required to prevent excessive current from being drawn.
The problem can be solved in part by inserting a positive temperature coefficient (PTC) thermistor in series between the ohms measurement current sourcing circuit and the instrument input terminals. Then, when a voltage higher than a predetermined value is presented, sufficient current will flow through the thermistor body as will cause its resistance to change from a relatively low value to a value of several hundred thousand ohms whereby excessive current flow is inhibited. However, thermistors are not instantaneous in their operation and an initial surge or influx of current can result before the Curie temperature is reached, i.e., before the thermistor transitions to a high resistance state. This initial influx current can be objectionable from the standpoint of the circuit across which the voltage is being measured. It would be desirable, then, to be able to measure voltages in a multipurpose instrument without large currents being drawn.